It's what you think, it has a fixed-resistance terminal somewhere between the two end terminals, and a terminal attached to a wiper as well. Just like it looks one the schematics. 4-terminal device. Maybe Uncle Ned has some?

Nah, one line entering the zigzag (resistor) has an arrowhead, one is just a line going straight in.

It's a way to get boost and cut I think, I am not sure exactly, not being a super-filter-guru.

You'd have to :

a) know the original tapping point;
b) have a pot of the correct value;
c) open it up without screwing it up;
d) figure out a way to connect a fixed terminal to the conductive track at the right point;

to make your own.

Look on www.geofex.com for an article on potentiometers in which RG speculates on how to do this.

It isn't really two pots in one. There's just an extra terminal, like it's connected to the junction point where you've soldered two resistors in series.

Take a close look at how a pot is constructed. You'll see three terminal lugs. The outer two can be seen mechanically connected to the ends of the resistance track, like they're riveted or crimped in place. The center terminal lug connects to a wiper that touches the resistance trace somewhere between each end. You can adjust where this point occurs, by turning the shaft. (Do yourself a favor. Find an old pot, a bad one that's been tossed is great, and take it apart to see the workin's. Bend back the crimp tabs around the edge of the metal case. It will come off, and everything else will become visible.)

Now, back in the early hifi days, some tone control circuits were designed where a deliberate neutral position was desired, for a repeatable flat frequency response setting ("tone controls electrically out of the circuit"). To accomplish this, an extra fixed terminal was mechanically connected somewhere along the resistance track. This was called a tapped potentiometer. It still has the wiper for adjustment, so ends up with four wiring connection points. The schematic symbol for a tapped pot should have an extra dot shown somewhere between the endpoints on the zigzag resistor symbol. This is a fixed terminal, and physically it will appear coming out of the side of the pot case opposite the three normally encountered terminals.

Another common use for tapped controls was on a specially compensated volume control, called a Loudness control. In general, it allowed increased bass response with decreasing volume control settings, to compensate for the human ear's nonlinear response with respect to sound pressure level (Fletcher-Munsen curves). You'll find a button on the front panel of your home stereo to engage the additinal circuit components attached to this "loudness tap" on the volume control. There do exist multiple tapped volume pots for precision loudness tracking, but the normally encountered single tapped version has a tap at the physical center of the resistance track, which is at 10%/90% electrical resistance point on the audio taper volume pot.

There are single knob tone control circuits on simpler guitar amps (like Musicmaster Bass and I think Ampeg AC-12?) that use a tapped pot with the fixed center tap as sort of a neutral reference, which coincides with the center of the shaft rotation. The control gives bass boost at one end and treble boost at the other, flat response in the center, where the wiper sits directly at the fixed tap's location on the resistance track. This allows a flexible single knob tone control, but requires a specially made pot.

The problem nowadays is that these specially ordered tapped pots are no longer made as a standard stocked part. The brown panel Fender amps had some unique tone control features which rely on a tapped pot. Someday I will try to find a reliable way to attach an extra terminal to the midpoint of a 3-terminal pot, since I'm sure I can't afford a special run of 10,000 pieces.